CRETACEOUS FORMATIONS. 77 



in advance of this. The splenial has the same shape as in other Ichthi/osa/rri, being a 

 longitudinal plate, with its lower margin bent outwards at a right angle ; this margin 

 forms the lower border of a great extent of the ramus, underlapping the dentary at 

 the situation of the posterior fracture, which is a little prior to the junction of the two 

 rami forming the symphysis ; it is withdrawn to the inner side of the dentary at the 

 anterior fracture. The ascending or vertical plate of the splenial forms the largest 

 part of the bone, and is much thicker than in the Crocodile with a jaw of the same 

 depth : its transverse diameter in the present Ichthyosaur is 3 lines. 



The dentary is a long bone which, at the hinder fractured part, (T. XVI, fig. 1, 32,) 

 appears as if it were folded lengthwise twice upon itself, forming a sigmoid transverse 

 section ; but the outer part of the bone increases in thickness as it advances forwards, 

 and the inner alveolar wall presents, at the anterior fracture, more the appearance of an 

 accessory plate or process sent off from the inner side of the body of the bone. The 

 vertical diameter of the dentary pieces is 2 inches 3 lines. 



The outer part of the dentary at the hinder fracture is 6 lines in thickness, smooth 

 and convex on its outer side, which is traversed by a longitudinal groove, y, which 

 also slightly narrows as it advances. The alveolar plate is continued downwards and 

 inwards at an angle of about 50°, diminishing in thickness as it descends, and again 

 increasing after it has risen, to form the inner w^all of the alveolar groove. The depth 

 of the groove is 1 inch 5 lines ; its width is 1 inch. At the anterior section, 5 inches 

 in advance, (T. XXVI, fig. 2,) the alveolar groove («c) has contracted to a diameter of 

 8 lines, and is 1 inch 2 lines in depth ; the inner alveolar wall, («/,) has increased in 

 thickness. 



The lower jaw, in the present fine fragment of skull, appears to have been broken 

 across just anterior to the meeting of the two rami, where they form the symphysis. 



What is wanting in the specimen of the Ichthyosaurus cavqyijlodon in Mr. Carter's 

 collection to give, ex visu, the proportions of the jaws of that species, is, in great part, 

 supplied by the fragments from Mr. Taylor's collection, which had been previously 

 discovered in the Grey Chalk near Dover. 



The hinder end of the portion of the left ramus in that specimen, which measures 

 2 feet 7 inches in length, has been broken away from the part which corresponds with 

 the front end of the portion of the same bone in Mr. Carter's specimen, and this end is 

 nearly 1 foot distant from the hindmost part of the same specimen. We thus gain an 

 extent of jaw by this addition of nearly 3 feet, and at least 1 foot more would be 

 required to complete the whole length of the jaw. 



Owing to the partial dislocation of the rami, the aspect of the alveolar groove is 

 more outwards than is natural ; but in the proper relative position it is turned more 

 obliquely outwards than that of the upper jaw, and the roots of the lower teeth, as 

 Mr. Carter has well remarked, present a curvature which compensates for the obliquity 

 of this alveolar groove, and gives a more vertical direction to their crowns. 



